Cryptosporidium parvum infections in immunocompromised individuals often develop into chronic, severe cryptosporidiosis that can become life-threatening. In conjunction with low CD4+ cell levels, other immune system factors are expected to contribute to infection chronicity in the immunodeficient host. Elucidation of immune responses and identification of features of immune dysregulation, such as cytokine abnormalities or inability of T-cells to proliferate in response to key cryptosporidial antigens, might identify patients at high risk or cryptosporidiosis. It is hypothesized that infection resolution in the immunocompetent host is linked to specific antigens responsible for the activation of lymphocyte populations and induction of cytokines. Consequently, a lack of response to these key antigens and development of certain cytokine profiles may lead to chronic, intractable infections. The overall goal of this proposal is to identify cryptosporidial antigens that are important in immune response and recovery. These antigens may be targets of antibody that block the attachment and penetration of invasive parasite stages, block fertilization in the sexual stages or may be targets of cell-mediated immune (CMI) responses. Recombinant antigens from a cDNA library, identified by their reactivity to specific anti-cryptosporidial antibodies and native antigen fractions will be used as the source of antigen. The applicants will establish the importance of each of these antigens by assessing their ability to elicit cellular immune responses using a mouse model. Since mucosal T-cells are thought to play a critical role in the immunity to this parasite, these antigens will also be evaluated for their ability to elicit in vitro responses from T-cells originating in the lamina propria and mesenteric lymph nodes as well as from IELs. The investigators will evaluate the various cytokines produced in response to these antigens in murine cell populations. Additionally, T-cell clones specific for 3-4 of these key antigens will be established and engrafted into infected mice. The ability of these cell lines will to reduce or clear infection will aid in determining if these antigens are important in response and in recovery. Successful use of prophylactic and treatment modalities requires an improved understanding of the natural immune mechanisms responsible for the control of the infection.